Titin is a giant protein found in vertebrate cardiac and skeletal muscle that spans the entire half sarcomere. Different parts of the titin polypeptide perform different functions. The N-terminal region of titin contains multiple binding sites for Z-disc proteins and appears to regulate Z-disc assembly. Titin's I-band region contains spring elements responsible for the elastic response of the stretched sarcomere. Titin's A-band region contains immunoglobulin (Ig) and fibronectin type 3 (Fn3) domains that are arranged in regular patterns that provide multiple binding sites for other thick filament proteins. This region of titin may serve as a molecular ruler in charge of assembly and length control of the thick filament. The approximately 200 KDa C-terminal region of titin anchors titin within the M-line lattice and contains a kinase domain with serine/threonine specificity. The function of the kinase domain is unknown. The titin kinase is highly conserved in vertebrates and the titin-like proteins found in invertebrates contain homologous kinase domains, suggesting that the titin kinase performs functions of critical importance. The long-term goal of this work is to establish the roles of the titin kinase in muscle function and muscle and cardiovascular disease. To lay the groundwork for this I have used gene targeting and developed conventional and Cre-inducible titin kinase knock-out mouse models. Furthermore, I will investigate a novel titin kinase binding protein named MURF. In the proposed research I will apply these new tools, using a multidisciplinary approach, and investigate the role of the titin kinase in muscle. My aim is to identify the titin substrate and study the relation between MURFs and titin kinase function. I will test the hypothesis that the titin kinase is critical for growth, division and survival of cells and that the kinase is important for in vivo muscle function.